The present invention relates to separation membranes and, in particular, membranes for the purification of hydrogen. The membranes involve a metal-based layer having sub-micron scale thicknesses, and methods for the preparation of these membranes and miniature devices incorporating these membranes are disclosed. In addition, the membranes can be used to effect chemical reactions such as hydrogenation and dehydrogenation reactions.
Much effort has been placed on developing synthetic membranes for applications such as (1) the separation of chemical components and (2) the catalysis of chemical reactions. Like their biological counterparts, synthetic membranes form an interface between two regions. One application for the membranes involves the separation and/or purification of gases. The capability to provide high purity gases is beneficial for certain devices such as fuel cells. The leading fuel cell technology involves a reaction between feed hydrogen and oxygen sources to generate power. As the purity of the hydrogen gas source increases, fuel cells generally display improved efficiencies because impurities such as carbon monoxide, which can poison fuel cells, are substantially eliminated from the feed.
Membranes can also be used to effect chemical reactions in a manner similar to heterogeneous catalysts. Membranes are distinguished from heterogeneous catalysts, however, in that membranes can also function as an interface between reactants and products or between impure component mixtures and the resulting purified component, and thus membranes can provide a dual catalyst/purification function. For example, the purification step can involve contacting an impure source of hydrogen gas with one side of a membrane layer and causing purer hydrogen gas to exit through the other side. By providing a reagent capable of undergoing hydrogenation on the other side, hydrogenation can occur with this purified source of hydrogen in the presence of a catalytically active metal surface. Dehydrogenation reactions can also occur at membrane surfaces, where membranes provide the added benefit of separating the hydrogen gas released from the reaction from the dehydrogenated product.
In addition, higher selectivity can be achieved with membrane catalysts. For example, typical hydrogenation reactions occur in the presence of a large excess of hydrogen gas. Because hydrogen transport through metal layers or membranes is diffusion-limited, the rate of hydrogen exiting the membrane can be maintained at relatively low concentrations. Thus, by using a membrane as a catalyst, a large excess of the reactant can be placed on the other side of the membrane layer, i.e., the side opposite a source of impure hydrogen gas, which can consequently react with a relatively low concentration hydrogen stream exiting through the membrane.
There exists a need to miniaturize membrane systems to produce more portable devices such as chemical devices which can include fuel cells or microreactors. Applications for such portable power generation systems span a wide range of technologies from military/defense to automotive. For example, in the U.S. Armed Forces, the operating power requirement for equipment of a dismounted soldier is outgrowing the capacity of batteries to supply that power, driving interest in man-portable chemical or hybrid power sources. In the case of the automotive industry, stricter emission limits, as well as concerns with greenhouse emissions are driving research into electric-powered cars. Most electric car designs include a miniature hydrogen generation plant combined with fuel cells to convert hydrogen into electricity.
One major obstacle in using devices such as fuel cells is the storage of hydrogen. Hydrogen storage for portable or mobile power sources presents a difficult challenge due to the low compressibility of hydrogen gas. In addition, large volumes of stored hydrogen gas can present a safety hazard. Some car manufacturers have diverted attention away from hydrogen fuel sources to methanol storage instead, and a number of small methanol reforming systems have been developed for use in cars. These systems can weigh about 200 lbs, however, which would not be practical for more portable applications.
Certain prior efforts to miniaturize membranes, i.e. provide membranes having smaller thicknesses, have involved porous membranes. Porous membranes are typically more fragile than solid layers. For example, U.S. Pat. No. 4,857,080 discloses depositing a metal on a microporous support to result in a layer having a thickness of less than 0.5 xcexcm. The layer can be strengthened with an overcoat xe2x80x9cto protect the fragile surface.xe2x80x9d U.S. Pat. No. 5,160,618 discloses porous inorganic membranes having thicknesses of less than 0.5 xcexcm for separating oxygen from oxygen-containing gaseous mixtures. U.S. Pat. No. 5,652,020 relates to hydrogen-selective membranes but the thicknesses achieved are between 10 xcexcm and 20 xcexcm. U.S. Pat. No. 5,734,092 discloses a macroscale palladium layer containing microfine channels that allow diffusion of hydrogen gas.
The ability to miniaturize membrane systems can eventually lead to new applications in membrane technologies and improvements in already existing technologies. Much work still needs to be achieved in providing thin membrane layers. One major obstacle lies in the requirement that in many cases membranes must be constructed to withstand pressure differences between an area of high gas pressure, e.g. the incoming gas source, and relatively low gas pressure, e.g. the outgoing, purified gas. Known membranes having micron-scale dimensions typically are unable to withstand the pressure differences required for gas separation or chemical catalysis applications.
The present invention provides a membrane incorporating a metal-based layer having sub-micron scale dimensions. Devices incorporating micromembranes are disclosed as well as methods for performing selective hydrogenation/dehydrogenation reactions and corresponding microreactors that can catalyze these reactions by using the micromembranes of the present invention.
One aspect of the invention provides a gas separation membrane comprising at least one support layer and a metal-based layer positioned adjacent the support layer. The metal-based layer has a thickness of less than about 1 xcexcm and is capable of performing gas separation or catalysis.
Another aspect of the invention provides an integrated gas separation membrane comprising at least one insulating support layer having first and second portions. The first portion is positioned adjacent a silicon surface. A metal-based layer is positioned adjacent the second portion of the support layer.
Another aspect of the invention provides a method for fabricating a gas separation membrane. The method comprises providing at least one insulating support layer and patterning holes in the at least one support layer. A metal-based layer is deposited adjacent this support layer, where the metal-based layer has a thickness of less than about 1 xcexcm.
Another aspect of the invention provides a method comprising providing a membrane having at least one support layer positioned adjacent a metal-based layer. The metal-based layer has an active surface and the metal-based layer has a thickness of less than about 1 xcexcm. The method involves chemically reacting at least one reagent at the active surface.
Another aspect of the invention provides a portable power generation system. The system comprises at least one gas separation membrane. Each membrane comprises at least one insulating support layer and a metal-based layer positioned adjacent the support layer. The metal-based layer has a thickness of less than about 1 xcexcm. The system also comprises at least one fuel cell integrated with the gas separation membrane. Another aspect of the invention provides a chemical microreactor. The microreactor comprises a membrane having at least one insulating support layer positioned adjacent a metal-based layer. The metal-based layer has a thickness of less than about 1 xcexcm. The metal-based layer has two opposing sides where each side is an active surface capable of catalyzing a chemical reaction. A source of a diffusing species is directed toward one side and a source of at least one reagent is also directed toward one side.
Another aspect of the invention provides a device incorporating a separation micromembrane. The device comprises a membrane having at least one insulating support layer positioned adjacent a metal-based layer. The metal-based layer has a thickness of less than about 1 xcexcm and has two opposing sides. Each side is an active surface capable of catalyzing a chemical reaction. The device also comprises a source of a diffusing species directed toward one side.
Another aspect of the invention provides an article comprising a membrane including at least one support layer and a metal-based layer positioned adjacent the support layer. The metal-based layer has a thickness of less than about 1 xcexcm and is capable of performing gas separation. The article also comprises a heating element positioned on the membrane.
Another aspect of the invention provides an article comprising at least one support layer containing fabricated holes. The holes have an average size of less than about 1 xcexcm. A metal-based layer is positioned adjacent the support layer, the metal-based layer having a thickness of less than about 1 xcexcm.
Other advantages, novel features, and objects of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings, which are schematic and which are not intended to be drawn to scale. In the figures, each identical or nearly identical component that is illustrated in various figures is represented by a single numeral. For purposes of clarity, not every component is labeled in every figure, nor is every component of each embodiment of the invention shown where illustration is not necessary to allow those of ordinary skill in the art to understand the invention.